1. Field of the Invention
The present invention relates to an optical pickup head used in a apparatus for recording, reproducing, or erasing information on an optical storage medium, and to an information recording/reproducing apparatus.
2. Description of Related Art
Optical memory technologies using optical storage media with a pit-shaped pattern, that are storage media of high density and large capacity, are increasingly adopted for digital audio disks, video disks, text file disks, and data files, for example. In recent years, high-density, large capacity optical storage media known as DVDs have been put into practical use and have garnered widespread attention as information media capable of handling large quantities of information, such as moving pictures. These DVD optical storage media are recorded and reproduced using a so-called red semiconductor laser that emits laser light of a wavelength near 650 nm.
A conventional optical pickup head in an optical disk system capable of recording and reproducing is described using FIG. 22.
A semiconductor laser light source 101, serving as the light source, emits a linearly polarized divergent beam 700 with a wavelength λ2 of 650 nm. The divergent beam 700 emitted from the semiconductor laser light source 101 is incident on a diffraction grating 510 and split into three beams of zero order, −1st order, and +1st order diffracted light. The zero order diffracted beam is a main beam 700a for recording/reproducing information and the +1st and −1st order diffracted beams are sub-beams 700b and 700c used in a differential push-pull method (hereinafter, referred to as DPP) to detect tracking error (hereinafter, referred to as TE) signals stably. The ratio of the diffraction efficiency of the zero order diffracted beam to either one of the 1st order diffracted beams is ordinarily set from 12:1 to 20:1, and here it is 20:1. Accordingly, the sub-beams 700b and 700c are prevented from affecting the main beam 700a, and unintentional recording on an optical storage medium 410 can be avoided.
The three beams created by the diffraction grating 510, that is, the main beam 700a and the sub-beams 700b and 700c, pass through a polarizing beam splitter 520 and are converted into parallel beams by a collimating lens 530 with a focal length of 15 mm. The parallel beams pass through a quarter wavelength plate 540 and are converted into circularly polarized light, after which they are converted into convergent beams by an objective lens 560 with a 3 mm focal length. The opening of the objective lens 560 is restricted by an aperture 550, and its numerical aperture NA is 0.6.
The optical storage medium 410 is provided with a transparent substrate 410a and an information recording plane 410b, and the thickness of the transparent substrate 410a is 0.6 mm. The convergent beam from the objective lens 560 passes through the transparent substrate 410a and is focused on the information recording plane 410b.
FIG. 23 shows the relationship between the tracks and the beams on the optical storage medium. As shown in FIG. 23, tracks, which are a plurality of continuous grooves, are formed on the information recording plane 410b of the optical storage medium 410 (FIG. 22). Tracks Tm−1, Tm, and Tm+1 are lined up in order, and the track pitch P2, which is the distance between the track Tm−1 and the track Tm and between the track Tm and the track Tm+1, is 0.74 μm. The beams are arranged such that when the main beam 700a is positioned on the track Tm, the sub-beams 700b and 700c are positioned between the tracks Tm and Tm−1 and the tracks Tm and Tm+1, respectively. Consequently, there is a 0.37 μm wide spacing L2 between the main beam 700a and the sub-beams 700b and 700c in the direction perpendicular to the track Tm.
The main beam 700a and the sub-beams 700b and 700c focused on the information recording plane 410b are reflected, and after passing through the objective lens 560 and the quarter wavelength plate 540 and being converted into linearly polarized light with a polarization that is rotated by 90° with respect to that of the incident path, they pass through the collimating lens 530 and are converged into convergent light. This convergent light is reflected by the polarizing beam splitter 520, passes through a cylindrical lens 570, and is incident on an optical detector 300. Astigmatism is imparted on the main beam 700a and the sub-beams 700b and 700c when they pass through the cylindrical lens 570.
The optical detector 300 has eight light receiving portions 300a, 300b, 300c, 300d, 300e, 300f, 300g, and 300h. The light receiving portions 300a, 300b, 300c, and 300d are for receiving the main beam 700a, the light receiving portions 300e and 300f are for receiving the sub-beam 700b, and the light receiving portions 300g and 300h are for receiving the sub-beam 700c. The light receiving portions 300a, 300b, 300c, 300d, 300e, 300f, 300g, and 300h each output a current signal corresponding to the amount of light received.
Using each of the signals output from the light receiving portions 300a, 300b, 300c, and 300d for receiving the main beam 700a, it is possible to obtain focus error (hereinafter, referred to as FE) signals through the astigmatism method, TE signals through a phase difference method, TE signals through a push-pull method, and information (hereinafter, referred to as RF) signals recorded on the optical storage medium. Also, when recording/reproducing continuous groove disks such as DVD-RW disks, TE signals can be obtained through DPP by jointly using the signals output from the light receiving portions 300e, 300f, 300g, and 300h for receiving the sub-beams 700b and 700c. After being amplified to a desired level and phase compensated, the FE signals and the TE signals are supplied to actuators 910 and 920, and based on these signals, focusing and tracking control are performed.
In DVDs, ROM disks for read only are standardized as two-layered disks provided with two information planes. Information can be read out from these two-layered disks without any problems by detecting the TE signals through the phase difference method using the conventional optical pickup head.
Moreover, at the research and development level, there have been many publications of research results for two-layered recordable disks having two information recording planes (hereinafter, referred to as two-layered recording disks). Initially, no information is written on two-layered recording disks, so TE signals cannot be detected by a phase difference method. For this reason, the TE signals are detected by DPP, as is the case with single-layered recordable disks.
However, even if two-layered recording disks are used with the above-mentioned conventional optical pickup head and TE signals are detected by DPP, there is the problem that letting the objective lens perform tracking generates an uncorrectable offset in the TE signals.
This is because when information is recorded/reproduced with one of the information recording planes of the two layers (hereinafter, that information recording plane is referred to as the focus plane), a portion of the beam forming a focal point on the focus plane is reflected and a portion passes through the focus plane and arrives at the other information recording plane (hereinafter, that information recording plane is referred to as the non-focus plane). This beam is out of focus on the non-focus plane and is reflected by the non-focus plane toward the optical detector. The beam reflected by the non-focus plane cannot be fully cancelled during detection of the TE signals by DPP due to aberration and variations in the amount of beam light, for example. For this reason, tracking with the objective lens leads to fluctuations in the amount that cannot be cancelled and an uncorrectable offset is caused in the TE signals.
This results in displacement from the track and partially erases information recorded on adjacent tracks when recording information to the optical storage medium, which causes the problem that information recoded on the optical storage medium can no longer be read out with fidelity.
It is an object of the present invention to provide an optical pickup head with which offset is not caused in the TE signals even when tracking with the objective lens in a case where a two-layered recording disk is used. It is a further object of the present invention to provide an information recording/reproducing apparatus using this optical pickup head.